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Magnetism
Origin of Magnetism An accelerated charge produces a magnetic field around it . In an atom , there are electrons revolving around the nucleus , generating magnetic field . At the same time , the electrons also spin about their own axis , producing more net magnetic field . If in an orbital , there are two electrons with anti-parallel spins , then the magnetic moments are cancelled . In a half - filled orbital , the magnetic moments are not cancelled and hence there is a net magnetic moment . Magnetic Pole Strength Pole Strength (m) has the unit A . Magnetic Dipole Moment M = m.2l S/i unit : Am M = nIA Magnetic Equivalence Magnetic Induction B= (µ0/4π) (m1/r2) (a) Point situated on the axial line (End-on position):- F = (µ0/4π) 2Mr/(r2-l2)2 In case of a magnetic-dipole, F = (µ0/4π) 2M/r3 (b) Point situated on equatorial line (Broad side-on position) F = (µ0/4π) M/(r2+l2)3/2 In case of a magnetic-dipole, F = (µ0/4π) M/r3 © Point situated anywhere:- F= (µ0/4π) M/r3 √1+3 cos2θ Direction, tan β = ½ tan θ Torque T = MBsinθ Potential Energy = U = - MBcosθ W = MB (cosθ1 – cosθ2) Bar Magnets Force between bar magnets with like poles facing each other F = μ0/4π 6MM'/ d4 The Magnetic Lines of Force inside a bar magnet is from South to North pole . When two bar magnets are lying mutually perpendicular to each other, then, M = √M12+M22 = √2 mpI Revolving Electron The ratio of magnetic dipole moment with angular momentum of revolving electron is called as gyromagnetic ratio . Gyromagnetic Ratio = M/L = 8.8 x 1010 C / kg magnetic moment = M = evr / 2 Magnetic Intensity Magnetic Intensity (H) is a quantity used in describing magnetic field strength of a field . Magnetization The net magnetic dipole moment per unit volume is called as Magnetization (Mz) . Domain Theory The region in which all the magnetic moments are aligned in the same direction are known as domains . Diamagnetism Silver , lead , silicon , nitrogen , sodium chloride , copper , antimony , gold , mercury , water , air , hydrogen , etc. 1) All the orbitals are completely filled . 2) The spins are paired and the magnetic field is repelled . The diamagnetic materials move away from stronger magnetic field to weaker magnetic field . 3) If a thin rod of diamagnetic substance is freely suspended in external magnetic field , it comes to rest with it's length perpendicular to the field . 4) In absence of external magnetic field , the net magnetic moment is zero . Paramagnetism Aluminium , Manganese , Chromium , Platinum , oxygen , sodium , calcium , lithium , copper chloride , tungsten , niobium , etc. 1) Atom contains unpaired electron ; i.e. their orbitals are not completely filled . 2) The spins do not cancel each other and the magnetic field is attracted . The paramagnetic materials move towards the stronger magnetic field . 3) If a thin rod of paramagnetic substance is suspended freely in a magnetic field , it comes to rest , parallel to the magnetic field . 4) In absence of external magnetic field , the dipole moments of atoms are randomly oriented , and hence the net dipole moment is zero . Ferromagnetism 1) Atom contains a large number of unpaired electrons , which results in large magnetic moment . 2) The ferromagnetic materials move towards the stronger magnetic field . 3) When ferromagnetic substances are kept in magnetic field , they get strongly magnetized in the direction of magnetic field . 4) In the absence of external magnetic field , these materials have a resultant magnetic moment . Curie Temperature The temperature at which the domain structure is destroyed and the ferromagnetic substance loses its magnetism , is called Curie Temperature . Above Curie temperature , ferromagnetic substances become paramagnetic . Curie Law Magnetic susceptibility is inversely proportional to temperature . Category:Physics